<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .layout {
        height: 500px;
        width: 500px;
        background-color: rebeccapurple;
        display: flex;
        justify-content: center;
        align-content: start;
        align-items: center;
        flex-wrap: wrap;
    }

    .middle {
        width: 100px;
        height: 100px;
        background-color: khaki;
    }

    .san {
        width: 0;
        height: 0;
        border-top: 50px solid red;
        border-left: 50px solid transparent;
        border-right: 50px solid transparent;
        border-bottom: 50px solid transparent;
    }
</style>

<body>
    <div class="layout">
        <div class="middle">middle:自适应</div>
        <div class="san">我是content的第二次提交</div>
        <div class="san">修改</div>
        <div class="san">再修改</div>
    </div>
    <script>

    </script>
</body>

</html>